Assistant Commissioner Neil Basu, the head UK counter-terrorism policing which is leading the investigation, said he was "unable to say" if the incident in Amesbury is linked to the poisoning of the Skripals on 3 March - but it is their "main line of inquiry".

Mr Basu said: "In the four months since the Skripals and Nick Bailey were poisoned, no other people besides Dawn and Charlie have presented with symptoms, but their reaction was so severe it resulted in Dawn's death and Charlie being critically ill.

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Media captionFootage shows Ms Sturgess in Salisbury days before her collapse

"This means they must have got a high dose and our hypothesis is that they must have handled a container that we are now seeking."

It is understood the couple each had nerve agent on one of their hands.

PM 'appalled and shocked'

Ms Sturgess, from Durrington, was rushed to hospital on Saturday 30 June after becoming ill at a house in Muggleton Road in Amesbury.

She has two sons, aged 19 and 23, and an 11-year-old daughter.

Theresa May said she was "appalled and shocked" by her death. Speaking to MPs in the House of Commons, she also paid tribute to the "dedication of the staff" at Salisbury District Hospital and thanked them for their "tireless work".

Defence Secretary Gavin Williamson told MPs: "The simple reality is that Russia has committed an attack on British soil which has seen the death of a British citizen.

"That is something that I think the world will unite with us in actually condemning."

Image copyrightPAImage caption
Tributes were left to Ms Sturgess, who has three children

Kremlin spokesman Dmitry Peskov said Russia was "deeply concerned" about the incident in Amesbury - but said accusations that it was involved are "absurd".

The UK government previously blamed Russia for the poisoning of the Skripals in March and say the nerve agent used was a type developed by Russia. Russian authorities denied any involvement in the poisoning.

Mr Basu said detectives will continue with their "painstaking and meticulous work to gather all the available evidence".

Analysis

Image copyrightReuters

By Gordon Corera, BBC security correspondent

A hundred detectives are already working round the clock to try to establish how Dawn Sturgess and Charlie Rowley were contaminated by Novichok.

Swabs taken from them in hospital revealed the highest level of contamination on their hands, leading to the theory they picked something up.

It may well be that it was Dawn who came into first contact and Charlie received secondary contamination from her but that remains to be confirmed. The priority now is working out what that item was and where it is now.

Officers have found a red Ford Transit van which Mr Rowley travelled in with three other men before falling ill. The vehicle has been sent to the government's military research facility Porton Down.

Three men who also travelled in the van have shown no sign of being exposed to Novichok.

Meanwhile, the bus that Ms Sturgess and Mr Rowley used the night before they collapsed has tested negative for Novichok.

Public Health England said the risk to the general public "remains low" but warned people not to pick up "any strange items such as needles, syringes or unusual containers".

They have also offered advice for people who may have visited one of five areas identified by police.

Those locations are Muggleton Road, Boots pharmacy and the Baptist church in Amesbury, and John Baker House and Queen Elizabeth Gardens in Salisbury.

Anyone who was there between 22:00 BST on 29 June and 18:30 on 30 June should continue to follow advice, including washing their clothes in a washing machine, keeping other items double-bagged and securely fastened if they are dry-clean only.

Wiltshire Police Chief Constable Kier Pritchard said the news of Ms Sturgess's death would "send a huge shockwave across our communities" but stressed that "the risk to the wider public remains low".

He praised residents for their "continual strength and resilience" during "these challenging times".

What happened when?

Friday 29 June

12:20 - Dawn Sturgess and Charlie Rowley are together at John Baker House, Salisbury. They leave to visit Salisbury and later go to Queen Elizabeth Gardens. They return to John Baker House at 16:20.

22:00 - The couple catch a bus to Amesbury. Police believe they then spend the night at Mr Rowley's home on Muggleton Road.

Saturday 30 June

10:15 - The South Western Ambulance Service is called to Mr Rowley's address as Ms Sturgess has been taken ill. Mr Rowley is also present at the address as Ms Sturgess is taken to hospital.

12:00 - Mr Rowley visits the Boots pharmacy on Stonehenge Walk in Amesbury, returning to his house half an hour later.

1:45 - He goes to the Amesbury Baptist Church on Butterfield Drive, returning home at 15:00.

18:20 - Mr Rowley is taken ill. The ambulance service is called to his address and he is taken to hospital.